twin 1999 502MPI's on 32ft Sutphen.
Having bad vapor lock issues but mine acts a little different than many of my friends . Mine has no VL until about an hour after stopping and will not begin to start up - zero fuel pressure. Bleeding schraeder valve on fuel rail into a rag even several times was not enough to vent the vapor. Had to put on fuel gauge and hold the gauge vent valve open for several seconds while cranking to get fuel to the rail. After starts once, never an issue till stop again for an hour or more. Most others I know starts but stalls after taking off.
I've been told to install a LP fuel pump on each motor, a kit sold by Mercruiser. I hate to put on more equipment like the two new LP pumps if I don't have to.

1. Rather than that, has anyone moved the requlator from the fuel pump on the COOL FUEL pump to the end of the fuel rail and put in a recirc line from the regulator either to the tank(there is no extra tank port) or, to the filter? Regulator on the cool fuel pump has a special flange so I would have to fabricate an adaptor but that is ok.
2. Even easier may be to put on a 12v timer with an on/off switch, to pressure up the fuel rail every 15 minutes or so after I turn off the motors since it seems to hold pressure for about 30-45 minutes before starting to bleed off, and if I turn the key on every so often to keep up the pressure up in the rail it seems to do better but not enough testing yet as the weather is not hot enough yet. This would be simple with no extra fuel pumps. Anyone done this or had experience with this?

Seems like a recirc line would be best. I dont see what 2 new LPpumps will do for you. Timer idea is cool but just a band-aid. Get the in fill line extra port from CP.

Hundreds of threads in this section. Use the search function for vapor lock.

PISMO 10.
The LP pumps just put enough liquid into the filter and to the primary elect fuel to fill it so it can pump up the system as it was before without the vapor lock happend.. THe electric pump apparently is just completely vapor locked and would not pick up the liquid fuel. This extra pump setup is what most everybody is pushing, but I think there's a better way. cheaper, just as easy, and not have extra junk on a boat.

My '99 502's act exactly as yours. Im going the return line route

VAPOR LOCK - What I have realized is that many fixes like the LP pumps and/or a return line will NOT keep it from vapor locking when its sitting parked for an hour after running it and the heat just builds up in the fuel system, but it should help purge it out quickly, and keep it from happening for those people that get it when idling.
In my case, I'm trying to come up with a solution to KEEP it from vapor locking, like the timer issue to keep 38psi or so on it, instead of letting it bleed down. I'm not sure but it is possible that the 38psi is still not enough pressure to overcome the vapor pressure of gas at those high temps in the compartment. I suspect the fuel rails and system get up to 125F or so and I need to find the vapor pressure of gas at those temps. I am going to check the temps of the fuel system next trip out.

I did a lot of research on this issue when I had my 27' with 502. It was terrible. ...and only after a long soak (1~2 hrs). It sucks to pay $400~$500 to fix this Merc issue, but the low pressure pump kit from Merc absolutely prevented vapor lock from ruining our day (after installation).

I have not heard of vapor lock issues reappearing after a return line has been installed.
If you want your boat to start every time, listen to others that have been there.
Otherwise you will be screwing around with it all summer

Easy Rider
Did it start immediately after sitting? I suppose maybe it did vaporize but with the LP pump it did start right away. If so then problem solved and I just need to bite the bullet(but got two of em to buy).
It seem like maybe you would have to crank it a few seconds to purge out the vapors. If not, it sounds like just need to go that way.

I may try to insulate the filter, fuel lines, and FuelCool pump/exchanger and try that as a last resort, and keep it pressured up and try this as a last resort before going to the two LP pumps. May take a few trips to find out though.
thanks for the info.
Like I tell other people.... there is nothing like experience..... respect it.